By Les Blumenthal | McClatchy NewspapersWASHINGTON — Using sophisticated seismometers and GPS devices, scientists have been able to track minute movements along two massive tectonic plates colliding 25 miles or so underneath Washington state's Puget Sound basin. Their early findings suggest that a mega-earthquake could strike closer to the Seattle-Tacoma area, home to some 3.6 million people, than was thought earlier.

The deep tremors, which humans can't feel, occur routinely every 15 months or so and can continue for more than two weeks before they die back to undetectable levels.

The instruments are detecting an inch or two of movement — known as "episodic tremor and slip" — as the Juan de Fuca plate grinds and sinks beneath the North American plate. Closer to the surface, the two plates are locked together. When they snap, scientists say, it could produce a massive 9.0 or greater earthquake and a tsunami.

By comparison, the largest earthquake ever recorded was 9.5 on the Richter scale, in Chile in 1960. The largest in North America was the 9.2 Great Alaska Earthquake in 1964, which killed nine people and spawned a tsunami that struck the Northwest coast. The 1906 San Francisco earthquake, which killed 750 to 2,500 people, was estimated to be an 8.2.

Whereas the scientists once predicted that a mega-earthquake would be centered just off the Northwest coast, now — using data from the tremors research — they say that it could be 30 miles or more inland, under the Olympic Peninsula, which lies to the west of Seattle and Tacoma across Puget Sound.

"The closer you are to the source, the stronger the shaking," said Steve Malone, a research professor emeritus at the University of Washington.

Exactly how much stronger, and how much more damage such a quake would cause in the Puget Sound area, hasn't been calculated, Malone said.

While there's still plenty of debate about the findings within the scientific community, and while they may not be consistent with the models that geologists have developed, state officials are aware of the latest studies.

"People are aware of the possibility," said John Vidale, a professor of geophysics at the University of Washington and the state's seismologist. "We haven't exactly calculated the impact, but bringing the fault closer (to metropolitan areas) could increase the shaking."

Scientists have spent years studying what's known as the Cascadia subduction zone, an area where the two tectonic plants collide that stretches roughly 600 miles off the coast of Northern California to southern British Columbia, Canada.

As the Juan de Fuca plate slides under the North American plate, the two can become locked. When plates become locked, pressure builds. The pressure is released in what scientists call a mega-thrust earthquake, which easily can be magnitude 9.0. The Sumatra-Andaman Islands earthquake the day after Christmas in 2004 was a 9.2 mega-thrust quake that produced a devastating Indian Ocean tsunami that killed an estimated 233,000 people in 11 countries.

The last mega-thrust earthquake along the Cascadia subduction zone, estimated at 9.2, was in January 1700. It produced a tsunami that reached Japan. Cascadia subduction zone mega-thrust earthquakes happen on average every 400 to 500 years, but they can happen as little as 300 years apart or as much as 800.

A mega-thrust earthquake would be different from those that shake the Northwest occasionally. A mega-thrust quake occurs right on the boundary of two tectonic plates, while other earthquakes occur along cracks in the plate. Vidale likened what's going on beneath the earth's crust to a bunch of blocks jostling around. Where the smaller blocks collide, you can have more standard-type quakes. Where the biggest blocks, the tectonic plates, collide, you have a mega-thrust earthquake.

Since the deep tremors were first detected 15 years ago, scientists have been trying to determine what was causing them along the Cascadia subduction zone. Eventually, they concluded that the tremors reflected the slippage of the Juan de Fuca plate under the North American plate.

"It's a burst of noise that can go on for up to 24 hours over a period of several weeks," said Herb Dragert, a geophysicist with the Geological Survey of Canada in Victoria, British Columbia, who was among those who first developed the theory.

What's unique about the deep tremors, which occur in an area stretching roughly from Olympia, Wash., to Canada's Vancouver Island, is that they reappear about every 15 months. While tremors have been detected elsewhere along the Cascadia subduction zone, none is as regular or as prolonged as those in the Puget Sound basin, Dragert said.

"Every 15 months it's like tightening the guitar string a little more," Dragert said. "You don't know whether it will take it beyond the break zone."

According to the timetable, episodic tremor and slip should be going on just about now. Instead, it came last spring, catching scientists by surprise. Malone said that some tremors were detected southwest of Olympia last week, but that it was too soon to determine whether they were part of a new episode or just isolated ones.

If all the energy associated with tremors over two weeks were released in 10 seconds, Vidale said, it would equal a 7.0 earthquake.

Similar deep tremors are being tracked at other subduction zones around the globe, including in Alaska, Japan, Mexico and Chile.

And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. 1 John 5:20

GeodynamicsEpisodic Tremor and Slip (ETS)Episodic Tremor and Slip (ETS) is the name given to a process that occurs deep below the Earth's surface, along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow sliding, one plate over the other, of a few centimetres over a period of several weeks, accompanied by energetic seismic noise, called tremor. Tremor is distinctly different from the seismic signals generated by earthquakes.

And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. 1 John 5:20

The good news: New research is giving scientists a better handle on when the big one might strike the Pacific Coast.

The bad news: It probably will be sooner than we thought.

"The amount of devastation is going to be unbelievable," says Rob Witter, coastal geologist with the Oregon Department of Geology and Mineral Industries. "People aren't going to be ready for this. Even if they are prepared, they are going to be surprised by the level of devastation."

Witter spoke last week about the latest in earthquake and tsunami studies -- it's Earthquake and Tsunami Awareness Month -- as part of state and local efforts to educate the public on preparing for a megaquake of magnitude 9 or more. Witter and James Roddey, spokesman for the state agency, also will give a public talk Tuesday in Newport.

Witter says scientists are now in wide agreement that there's a 10 to 14 percent chance a powerful earthquake and tsunami will strike the Oregon coast in the next 50 years.

Why do they think that? The latest findings come from Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Lab at Oregon State University, Roddey says. But to understand Goldfinger's work, Roddey goes back to what he calls "one of the great scientific detective stories of the 20th century."

Twenty-five years ago, scientists didn't think the Cascadia subduction zone -- a place 50 to 75 miles off Oregon where two of the Earth's plates meet, one sliding under the other -- could produce earthquakes.

That changed after Brian Atwater, a renowned geologist with the U.S. Geological Survey, began in the mid-1980s to study earthquakes, eventually tracing a theory all the way to Japan, Roddey says.

Atwater studied a "ghost forest" -- dead trees in a tidal marsh -- by the Copalis River in Washington, and theorized that a megaquake had killed the cedars. But he needed more evidence.

That came when Japanese researchers published a paper in 1996 about a tsunami that struck Japan on Jan. 27, 1700. Atwater was so intrigued that he learned the language and traveled to Japan.

In 2003, he and a team of Japanese researchers published the book "The Orphan Tsunami" about their theory that the last megaquake struck the Oregon coast Jan. 26, 1700, spawning the tsunami that washed over Japan hours later.

"This was the final puzzle piece that convinced everyone we had the potential for great earthquakes," Roddey says. Work to understand just how great has gone on ever since.

Enter Goldfinger, the OSU researcher. "Chris has been able to go back 10,000 years, and by studying offshore landslides, he found that we've had about 20 of these magnitude9-plus subduction zone earthquakes where the whole 600-mile-long fault ruptures," Roddey says.

Then he found 18 other quakes magnitude8 to 8.5 in the southern part of the subduction zone. "So instead of 20 subduction zone earthquakes, we're up to 38," Roddey says.

And, Witter says, instead of thinking giant quakes occur every 500 years, Goldfinger's work puts the cycle at 300 to 350 years.

"We've almost doubled the probability of these events happening," Roddey says. "It just gives a whole lot more urgency to educate the folks at the coast that, 'Hey, you got to get ready.'"

Native Americans, Roddey says, have known this for centuries. Inhabiting the coast for 10,000 years, they passed stories of big quakes from generation to generation.

"They created a cultural tradition by retelling these events and legends," Roddey says. "We see that around the world. When the Sumatra earthquake struck (the Indian Ocean), the Andaman Islands were right in the middle of the rupture zone. There was huge ground shaking, but very few people died from the tsunami because they had also created this culture of awareness. They went to high ground. They survived the event."

Though the Sumatra and Cascadia subduction zones differ, Witter says, a tsunami here would be much the same.

"The geology and numerical models predict tsunamis could reach as high as 80 to 100 feet in Oregon, which is similar to the tsunami that struck Sumatra," Witter says. "We need to be very cautious and prepare for that event. It may not happen in a person's lifetime, but if it does, it's going to be equivalent to a Katrina-like event."

That doesn't mean we need to live in fear, though.

"Once you get prepared and you know where high ground is, once you've talked to your family about an emergency plan and talked about all those kinds of things, you've done your homework. You don't have to think about it again."

And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. 1 John 5:20

And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. 1 John 5:20